A tire monitoring apparatus is known in the art, in which an air pressure of each tire is transmitted by a radio wave from an air pressure sensor and a transmitter arranged in the wheel of each tire, the radio wave is received by a receiving antenna arranged within each wheel house, and the information containing the air pressure is notified to a driver by a meter within a vehicle room.
For example, as disclosed in Japanese Patent Publication No. 2002-257661, a tire monitoring apparatus has plural transmitters arranged in each tire and plural receiving antennas arranged in a vehicle body, so as to respectively correspond to the transmitters. The apparatus further has plural level changing means for the respective receiving antennas to reduce the level of a voltage signal induced by the receiving antenna, a coupling means for forming one synthesizing voltage signal by synthesizing voltage signals received from these level changing means, and a control means for controlling the operation of the level changing means.
In the above tire monitoring apparatus, the control means automatically discriminates, from which transmitter the receiving antenna has received the information contained in the received radio wave, based on an assumption that the radio wave from the transmitter is mainly received by the corresponding receiver.
The discrimination in the above apparatus is performed in the following manner.
When one of the transmitters transmits the radio wave, this radio wave is received by each of the plural receiving antennas. And each receiving antenna induces a voltage signal according to electric field intensity of the received radio wave. The respective voltage signals outputted from each receiving antenna are made to one synthesizing voltage signal by the coupling means through the level changing means for the respective receiving antennas.
At this time, the plural receiving antennas are located at different positions from each other with respect to the transmitter transmitting the radio wave. Therefore, the magnitudes of the voltages induced by the respective receiving antennas are different from each other. The voltage induced by the receiving antenna nearest to the transmitter transmitting the radio wave is largest.
In the above tire monitoring apparatus, when one of the transmitters transmits the radio wave and the synthesizing voltage signal is produced, one of the level changing means is operated to perform a level reducing operation, so that the synthesizing voltage signal is reduced by a voltage signal of such level changing means. A reduced amount of the synthesizing voltage signal is memorized in the control means.
The above operation of the level changing means is respectively performed in a sequential order for the four receiving antennas with a certain time intervals, and the respective reduced amounts are compared to determine which voltage signal of the receiving antennas has the largest reduced amount and thereby to determine which transmitter has transmitted the radio wave.
Thus, the control means can automatically determine from which transmitter provided in the tire the receiving antenna has received the information contained in the radio wave.
In the above tire monitoring apparatus, when one of the transmitters transmits the radio wave, one synthesizing voltage signal is formed by synthesizing the voltage signals received from the level changing means by the coupling means. And the control means determines from which transmitter provided in the tire the receiving antenna has received the information contained in the radio wave, by a degree of reducing the level of the synthesizing voltage signal.
As a result, the respective transmitters are recognized only one by one, and thereby it takes time until all the transmitters are recognized. Furthermore, since the coupling means is necessary, the number of constructional elements of the tire monitoring apparatus is large and the coupling means is a factor of an increase of manufacture cost of this apparatus.